When you first start using Unix based systems you quickly learn about the wonderful world the Unix command shell. One of the first commands you also learn about with "ls", "cd" and "cat" is "man". Man is short for manual and almost all unix commands come with a man page. Manual pages are usually quite informative about what the command does and all the options, but sometimes it's really hard to sort out. This can lead to trouble and frustration (boo!)
If you're anything like me you might occasionally have a problem with your computer and someone will tell you to run some command with a bunch of flags.. say "iptables -A INPUT -i eth0 -s ip-to-block -j DROP " or "rm -rf /" now you could run this run this or you could pass it to explain shell to help you sort out exactly what a command does. This is also really useful for commands where input is piped from one command to another.